In a direct-tandem color laser printer disclosed in Japanese patent publication no. 2006-98776, after a pair of registration rollers has corrected skew in a sheet conveyed from a feeding tray, the corrected sheet is then conveyed to a drum unit (image forming unit) including a plurality of photosensitive drums.
In the above-mentioned invention, the drum unit is designed to be removable from the main casing, thereby enabling the photosensitive drums to be removed all at once from the main casing. Thus, this configuration facilitates maintenance of the drum unit.
Generally, the registration rollers are designed to correct skew in recording sheets, such as sheets of paper, as described above. The registration rollers are, therefore, preferably disposed in as close proximity to the drum unit as possible.
This is because, if the registration rollers are disposed in a position far away from the drum unit, a recording sheet may become skewed again before reaching the drum unit, even though the registration rollers had previously registered the sheet before conveying the sheet downstream. Another reason is that it is difficult to control the timing at which the registration rollers supply a recording sheet to the drum unit with a high degree of accuracy.
On the other hand, if the registration rollers are disposed in proximity to the drum unit, the registration rollers and the drum unit may interfere with each other when the drum unit is being removed from the main casing simply by being pulled out in a horizontal direction. In such a case, it may not be easy to remove the drum unit from the main casing.
In the invention disclosed in the above-identified publication, therefore, one of the registration rollers located closer to the drum unit is provided in the drum unit. According to this configuration, when the drum unit is being removed from the main casing, one of the registration rollers is removed together with the drum unit, thereby preventing interference between the registration rollers and the drum unit from occurring.
As stated, a pair of registration rollers is provided for correcting skew in recording sheets, and, hence, one registration roller must be positioned relative to the other registration roller with a high degree of accuracy. However, as in the above-identified reference, when one of the registration rollers is configured to be removed together with the drum unit and the other registration roller is fixed to the main casing, it is difficult to achieve high positioning accuracy between the pair of registration rollers.
Moreover, although another method may be employed to prevent interference between registration rollers and a drum unit by incorporating one of the registration rollers in a cover that opens/closes an opening for removing/mounting the drum unit, high positioning accuracy between the pair of registration rollers is still difficult to achieve, because one of the registration rollers is fixed to a movable unit while the other one is fixed to a main casing, as in the invention identified above.
Further, high positioning accuracy between the registration rollers may be achieved even in the above-mentioned method if the movable, open/close cover is given high rigidity. However, this solution inevitably increases the weight and dimensions of the open/close cover, leading to a larger image forming device.
In view of the forgoing, it is an object of the present invention to provide an image forming device that can be made compact and can realize easy removal of a drum unit while achieving high positioning accuracy between a pair of registration rollers.